Perforating film by electrical discharge



H. G; SCHIRMER PERFORMING FILM BY ELECTRICAL DISCHARGE Oct. 7, 1969 Original Filed Aug. 26, 1964 ELECTRODE r e m U V. m h m T C r U s m B I r m G 8 MN I n m 5 v. L B U 4 0 T0. 6 I 4 F 4. m 5 6 3 A N o R 0 8 C 4 2 5 N o nu T B nun A L U S m I. M 2 l United States Patent O 3,471,597 PERFORATING FILM BY ELECTRICAL DISCHARGE Henry G. Schirmer, Spartanburg, SC, assiguor to W. R. Grace & Co., Duncan, S.C., a corporation of Connecticut Original application Aug. 26, 1964, Ser. No. 392,168, now Patent No. 3,348,022, dated Oct. 17, 1967. Divided and this application June 27, 1967, Ser. No. 662,228 Int. Cl. B29d 7/00, 7/20; H05b 7/16' U.S. Cl. 26425 ABSTRACT OF THE DISCLOSURE Longitudinally and transversely spaced perforationt He produced in a film of dielectric material by applying a high value potential between spaced apart electrodes to produce a corona, fixing a discontinuous dielectric material about one of said electrodes to provide areas for spaced concentration of electrical energy in a direction transverse to the movement of the film, said concentration being sufficient to perforate said film, passing said film through said corona, and periodically disrupting said corona to longitudinally space said perforations.

This application is a division of my prior copending application Ser. No. 392,168, filed Aug. 726, 1964, now Patent No. 3,348,022.

This invention relates to a method for perforating film by electrical discharge.

It has previously been proposed to improve the printability of polyolefins, such as polyethylene, by subjecting said films to high voltage electrical discharge. In a prefered prior process, the film is passed between a pair of electrodes having an extended surface area to which is applied a high alternating potential sufiicient to produce a diffuse corona between the electrodes. The corona is caused by partial breakdown or ionization of the atmosphere around an electrode. The electrodes must be so spaced that the film surface is exposed to the corona. The electrodes may comprise a pair of flat plates positioned parallel to one another. The electrodes may also comprise a drum having a stator spaced apart and concentric therewith. There may also be positioned between the electrodes a sheet of dielectric material to prevent "an areover and damage to the film being treated in the event that said film has pin holes or other weak spots therein. The dielectric covering also prevents pitting of the electrodes and helps to spread the corona over the entire width of the electrode and cause the film to be a minor portion of the total dielectric in the gap. Suitable dielec tries for ground roll coverings are grass, Mylar, epoxy resins and elastomers, such as chlorosulfonated polyethylene, silicon rubber and the like, and anodized coating. The elastomers are generally preferred since the only maintenance required is that it be kept free of any surface iregularities to prevent treat-through. This is a particularly vexatious problem when it is desired to surface'treat only one side of the film. It has been found that any space between the film and the dielectric cover will allow corona to form on the back of the film thus treating both surfaces.

It has now been found that this problem can become an advantage when certain conditions are applied so that a pattern of perforations is produced in the film.

Although polyolefins such as polyethylene have numerous uses as a packaging material, one of its assets, gas impermeability, is a detriment when packaging commodities which must breathe or which release a gas which must be removed from the container.

3 Claims "ice Numerous methods have been proposed for perforating film such as described in the patents to C. H. Schaar, US. 3,012,918 and US. 3,038,198. These methods involve passing the film over and in direct contact with a cooled perforated roll while subjecting the opposite film surface to a hot flame bath.

It is an object of the invention to provide a method for perforating film.

Yet another object is to simultaneously perforate and corona treat film.

These and other objects of the invention will be readily apparent to those skilled in the art from the disclosure, drawings and claims.

These objects are broadly accomplished by passing film through a corona formed by an electrode which is transverse of said film, providing a substantially uniform space between the opposite side of said film and a second discharging electrode, applying an alternating high value electrical current across said electrodes, providing for areas of concentrated electrical energy along said electrode which are transverse said film, said areas having sufficient energy to perforate said film with little or no corona in between said areas, thus providing transverse spacing for said perforations and periodically disrupting the electrical energy thus providing the longitudinal spacing for said perforations.

In one embodiment a uniform longitudinal spacing is provided by a foraminous dielectric material having trans verse members which move longitudinally through the space between the film and the discharging or ground electrode thus periodically disrupting the discharge of electrons.

In another embodiment the longitudinal spacing of the perforations is provided by pulsing the electrical current supplied to the charging electrode.

In another embodiment the transverse spacing of the perforations is provided by electrically insulating portions of the charging electrode, the areas of concentrated corona, and thus the concentrated flow of electrons, being at the perimeter of the non-insulated portions of said electrode.

It has now been surprisingly found that a discontinuous insulative dielectric material placed between the film and the discharging electrode permits perforation of the film. Particularly suitable materials are a fiber glass screen and a polyethylene netting. The size and shape of the openings in the spacer are not limitative and must only be sufiicient to provide support for the film and to provide a substantially uniform distance between the film and the discharging or grounded electrode. Preferably the spacing material is a dielectric net-like structure which periodically disrupts the flow of electrons. This distance is preferably between 0.01 and 0.2 inch. It has also been found that by varying this distance the pattern of the perforations is variable along the charging electrode or transverse to the moving film. In general, the transverse spacing of the perforations is proportional to the gap distance between the film and discharging electrode. For example, under certain known conditions, a inch thick screen results in a 4; inch spacing, two inch screens on top of each other result in a A inch spacing, and three inch screen result in approximately /3 inch spacing.

It is also possible to obtain transverse spacing of the perforations by simply employing an electrode having peaks from which the electrons are emitted. The transverse pattern will thus depend on the arangement of the peaks. For example, a sintered electrode is satisfactory although there will not necessarily be a hole coresponding to each peak.

Longitudinal spacing of the perforations in moving film is readily attained by periodically disrupting the corona or the areas of concentrated electrical energy causing the perforations. Although the invention is not limited to any theory of this surprising phenomenon, it is believed that the use of a dielectric spacing member having transverse members which periodically pass between the discharging the electrode and the film causes a disruption in the corona. It is believed that the discharging electrons must also follow the hole for a brief distance until the resistance is such that it is easier for the electrons to form a new hole than to cover the extending distance through the air to follow the hole. This disruption can also be achieved by simply pulsing the electric current such as by use of a distributor. In this case the spacing means need not have the transverse members but need only provide open space between the film and the discharging electrode to permit perforation.

Preferably the frequency is in the range of to 1000 kc., more preferably 100 to 300 kc. The voltage and current are variable over a wide range and are sufficient to provide a corona discharge for the gap employed between the film and the charging electrode. Suitable voltages are in excess of 100, preferably 500 to 10,000 kv. Suitable currents are in excess of .7 amp., preferably 1 to 1.5 amps. The space between the two electrodes is generally less than A", preferably to inch, although this depends primarily on the voltage.

Numerous shapes and types of electrodes have been employed for corona treatment. Any electrode shape or size may be employed herein which will produce a corona between the electrodes over the transverse portion of the film desired to be perforated. A suitable charging electrode is a simple piece of 18 gauge black iron about 2 inches wide and of the required length. The discharge end should be cut in a good sheet metal shear and sanded to remove burrs. Using nylon bolts to prevent stray corona, the strip is preferably bolted to a piece of electrical grade micarta and mounted at a proper distance from the grounded electrode roll. The grounded treating roll is connected to the ground on the generator terminal.

The invention is best described with reference to the drawings. FIGURE 1 represents the perspective view of a typical apparatus for perforating film by corona treatment of thermoplastic film such as polyethylene or polypropylene. In this type of apparatus, which is shown as enclosed within a framework 2, but which may employ any suitable frame structure, is a grounded steel cylindrical electrode 4 mounted on a shaft 6 driven by any suitable driving means such as motor 8. The opposite end may be mounted onto the framework by any suitable means such as a journal box 10. The exterior surface of the grounded electrode 4 is preferably, but not necessarily, covered with an insulating substance 12, such as rubber. Although the direction of rotation is not important, the grounded electrode is shown as rotating in the clockwise position looking from the motor end of the shaft. Immediately surrounding and in direct contact with the insulated grounded electrode is a spacing member 14 which preferably has transverse members at least as wide as the film and more preferably is a foraminous or netlike material, such as polyethylene netting having a substantially uniform thickness. This discontinuous material provides a uniform air space between the insulated grounded electrode and the film 16 which passes over the netting and in direct contact therewith. The film may enter the corona treating area by any suitable means such as by passing through a slot 18 in frame 2, then under a guide roller 20 mounted by any suitable means (not shown), then over the netting and under a second guide roller 22 so as to provide uniform and intimate contact between the film and the netting. Mounted directly above the grounded electrode is an area electrode, such as aluminum foil 24, which may be attached to a blade electrode shown by dotted line 26. If the aluminum foil electrode is placed at a very small distance from the film such as less than A inch, a corona will be formed in the space between the foil and the film. The aluminum foil electrode 24, or blade electrode which is attached thereto, is connected to a power source (not shown) through electrical conduit 28. The grounded electrode is grounded through conduit 30. A rubber weighting material 32 is positioned on top of the foil with an insulative plate 34 thereabove although neither of these are required since the foil is attracted to the ground roll by electrostatic forces. The passage of the film through the corona results in perforations being formed therein. This results in a uniform spacing or pattern of perforations as shown in FIGURE 2 which is an exaggerated enlargement-- of a section of perforated film 16. FIGURE 3 employs essentially the same equipment as shown inFIGURE 1, except for the use of a partially insulated blade electrode 36, Where there is no difference in functionality identical reference numbers have been employed for all the figures. In this illustration, the disruption of the discharging electrons isproduced by pulsing of the current by passing the current from a source of power (not shown) through a distributor 38 and an electrical conduit 40 to an elongated blade electrode 36. The film 16 enters through slot 18 as hereinbefore shown'but since direct contact with the partially insulated blade electrode 36 is desired in this instance there is virtually no clearance between the electrode 36 and film 16. The spacing between the film and the charging electrode, whether blade or area, is not limitative. The guide roll 42 is below the film and guide roll 46 is above the film. As best shown in FIGURE 4, which is an elevated view through 44 of FIGURE 3, the blade electrode 36 preferably comprises a rectangular elongated member enclosed in insulative material 48, such as electricians tape with portions of the underside, that is the side in direct contact with the film, having no insulation in specifiedareas to provide the desired spacing. The film 16 passes directly through the discharging electrons emanating from electrode 36 resulting in perforation of the film to cause orifices 52. The corona appears to concentrate at the edge of'th'e noninsulated portions and thus makes two perforations for each of said portions. Towers of corona 54 result in the uniform space between the insulation 12 of the grounded electrode 4 and the film 16 The invention is broadly applicable to perforating any film including all organic thermoplastic and thermosetting resins such as but not limited to polyolefins, including polyethylene, polypropylene, polybutene-l and the like, polyvinyls, vinylchloride copolymers, polyarnides including ny- Ion and the like. The term polymer as employed herein includes homopolymers, copolymers, terpolymers, block copolymers, laminates and the like. The film can be molecularly oriented. Film thickness which can be perforated depends on the voltage, distance, etc., but is preferably between 0.1 and 20 mils.

The invention is best illustrated by the following examples.

EXAMPLE I Holes were produced in several different types of film by means of a concentrated high frequency-high voltage current pulsed by a distributor. The electrical energy was first concentrated by alternately masking the blade electrode of a Lepel model HFSC-Z treater with electricians tape. Taped sections were /2 inch in length .and untaped sections were inch in length along the blade. When a piece of polypropylene film was held against the prepared electrode to provide an air gap of inch between the film and the rubber insulated ground roll, the concentrated electrical energy burned two holes into the film at each unmasked section of the blade. The two holes were formed in the film at the end of each unmasked section of the blade where the insulation tape provided a distinct border.

EXAMPLE 11 The blade electrode of Example I was replaced by an area electrode 5 inches long and 18 inches wide consisting of aluminum foil. The rubber insulated ground roll was wrapped with a polyethylene netting inch thick in order to provide an air gap between the foil electrode, the contacted film and the ground roll. After conforming both the electrode and film to the shape of the wound insulated ground roll, the roll was rotated at 50 f.p.m. and the film was passed between the netting and the area electrode and through a visible purple corona. The resulting film was punctured with a myriad of tiny holes spaced approximately every /2 inch along the width of the film. The gap distance was found to affect the hole spacing. Moving the electrode nearer to the ground roll caused the holes to be spaced closer together and moving the electrode away from the ground roll caused the holes to be spaced further apart. The size of the hole produced in the film was also found to vary inversely to the speed of film passage through the treating area.

The table below illustrates the power settings used to produce holes in various types of film using the area electrode.

While certain examples, structures, composition and process steps have been described for purposes of illustration, the invention is not limited to these. Variations and modification within the scope of the disclosure and the claims can readily be effected by those skilled in the art.

I claim:

1. A method of producing longitudinally and transversely spaced perforations in a film of dielectric material comprising:

(1) providing two spaced apart electrodes, one being a charging electrode and the other being a grounded electrode, said grounded electrode being elongated and located transverse to the direction of movement of said film;

(2) providing an insulating cover over said grounded electrode;

(3) fixing a discontinuous dielectric material about one of said electrodes, the discontinuities in said material providing areas for spaced concentration of electrical energy at least in the direction transverse to the movement of said film;

(4) producing a corona between said spaced apart elec.

trodes by applying a high value potential therebetween, the energy concentrated in the areas provided by said discontinuous material being sufiicient to perforate said film;

(5) passing said film through said corona in the space between said discontinuous material and the other electrode; and,

(6) periodically disrupting said corona to longitudinally space said perforations.

2. The method of claim 1 wherein the discontinuous dielectric material in step (3) is provided by wrapping a net-like dielectric material around the covered grounded electrode thereby periodically disrupting said corona to provide longitudinal spacing of the perforations as set forth in step (6).

3. The method of claim 1 wherein:

(a) the discontinuous dielectric material of step (3) is provided by applying a dielectric material to the charging electrode to provide transversely spaced apart areas for concentrating electrical energy; and,

(b) the periodic disruption of the corona in step (6) is by pulsing the current to the electrodes which produce the corona.

References Cited UNITED STATES PATENTS 2,365,576 12/ 1944 Meaker.

2,385,246 9/1945 Wilsey 219384 2,528,158 10/1950 Menke 219384 2,678,373 5/1954 Suran 219384 2,859,480 11/1958 Berthold 26422 X 3,098,143 7/1963 Warnt 219384 3,167,641 1/1965 Parmele 219384 3,206,590 9/1965 Cox 219384 3,391,044 7/1968 Kaghan 264-22 X OTHER REFERENCES Heide and Wilson, Guide to Corona Film Treatment In Modern Plastics, vol. 38, No. 9, May 1961, pp. 199-206, 344.

ROBERT F. WHITE, Primary Examiner S. I. LANDSMAN, Assistant Examiner 

